TRUTHS & MISCONCEPTIONS ABOUT BATTERY PACKS – PART 1

We are looking again, 12 months on back at this two part series. We discuss some points which will help you understand the difference between a good and bad lithium battery:

General Reading About Battery Packs

Over the last few years we have seen a huge increase in the amount of batteries commercially available for electric bikes (and other light electric vehicles). The bulk of these packs contain 18650 cells with various chemistry, voltage and capacity arrangements. Not all chemistry types are the same, nor is the quality of the 18650 cells from different manufacturers. What we are witnessing is very much an industry in its infancy with no directive or standards. Because of this its important to be educated about your buying decisions. There are four main things which contribute to whether a battery will last its intended life, or fail prematurely costing its owner money and / or worse.

  1. Does the pack have adequate thermal management? (not BMS, but related).
  2. Does the battery have quality electrical components? (cells and battery management system).
  3. Is the pack spec’d and suited to the controller / motor you intend to pair it with?
  4. Does the pack have adequate safety and built-in-redundancies to avoid thermal runaway and other catastrophic events?

Image: 18650 cells on the workbench

Its important to understand the different chemistry in common Lithium-Ion packs. Anything to avoid? And what is considered best use for the different chemistry types?

  1. Li-Ion and its chemistry

Lithium-Ion refers to the movement of lithium ions from the negative electrode to the positive electrode during discharge and vise-versa. This process is present in all Li-Ion 18650 cells.

However, chemistry differs depending on manufacturer and cell application. There are many types of cell chemistries used by big cell manufacturers. Here are a few common chemistry types used in ebikes:

  • Lithium Maganese Nickel ICR – NCM (LiNiMnCoO2). Used in higher high discharge applications. Manganese is used for low resistance and safety due to heat reduction properties. Nickel in the chemistry provides the higher power.  These cells are quickly have been used widely in ebike packs (such as the Samsung 22p), but the market is trending towards NCA.
  • Lithium-Aluminium INR – NCA (LiNiCoAlO2). This is a very common chemistry widely used by Panasonic (18650PF and 18650B, popularised by Tesla). Also used in the latest gen Samsung and LG Cells. These batteries tend support lower discharge currents but offer high capacity and increased lifecycles. This chemistry is very stable and resistant to shock making it a good choice for electric bikes.
  • Lithium-Cobalt Oxide ICR-LCO (LiCoO2). This chemistry delivers high energy density but can be unstable. This chemistry is popular as it is cheap to produce. Common in cell phone batteries and cheaper cells common in ebikes like the Samsung 26F. Make sure you have a reliable BMS!
  • Lithium Iron Phosphate (LiFePO4). This chemistry is know to be the most stable of all lithium chemistries but tends to self-discharge quickly, is heavy and becoming less common. Lifecycle rating is commonly >1500 cycles.

What about Lithium Polymer? 

Lithium polymer gets a special mention here not because of DIY enthusiasts using hobby grade Lipo pouch cells in series / parallel but because of high-end pouch cells from companies like Kokam, who are getting incredibly high performance and energy density (upto 265Wh/kg) from a small lightweight pack. These packs are becoming popular in very high-powered electric bikes, motorcycles and cars. Kokam is also being used in grid-scale storage batteries and Australia’s very own Byron Bay Solar Train.

Image: The Kokam super-cell

The importance of the pack build: Structural Rigidity, Heat and Current Transfer

Cell Holders or Not?

Using good quality, stable lithium cells in your battery is only a start. There is a number of building techniques that can be the difference between a reliable and safe pack that will last its rated life-cycle and one which is unsafe and performs poorly. There is a growing trend among vendors to cram as many cells as they can into a small frame pack, often compromising build techniques to do so. These vendors claim attractive specifications, such as 52v (14s) 14ah (4p with a 3500mAh cell) which fits inside these small cases which are designed for 52 cell only with cell holders. They get around this by removing the cell holders and gluing together the cells fitting 56 cells (14s4p) or as much as 70 cells (14s5p) into these small frame packs. Its safe to say that the majority of ebike packs on the market are poorly designed, with importers putting low cost over safety, thermal performance and electrical standards.

Image: A overloaded Hailong Pack – should be avoided. Credit electricbike-blog.com. 4 parallels with this cell should only be used for discharge applications upto 20a. 

Removing the cell holder reduces the structural rigidity of the pack and decreases the amount of heat dissipated between the cells. Cheap hot glue will also melt if the cells hit peak temperature (likely in this design) of 60 or 70 degrees. This is only the start of the problem.

High Capacity vs High Discharge

The Hailong packs above are advertised as supporting upto 60a continuous. However, looking at the cell specifications they are using low discharge cells (Panasonic 18650b, Sanyo GA). It has been proven that these cells heat up considerably at discharge rates over 5a per cell. A 4 parallel pack with these cells therefore should only be pushed to a maximum of 20a. If you push these cells past this it will lead to early life failure. The problem is compounded for the cells in the middle of the pack which are susceptible to higher levels of heat stress. Performance wise these packs sag horribly at anything over 20a discharge. Sag is defined as follows:

“Voltage sag (U.S. English) or voltage dip (British English) is a short duration reduction in rms voltage which can be caused by a short circuit, overload or starting of electric motors. A voltage sag happens when the rms voltage decreases between 10 and 90 percent of nominal voltage for one-half cycle to one minute.”

If you have a voltage sag problem its a good indication that you do not have enough parallels in your pack and / or are using cells that are not suited to your controller / motor setup. Current needs to be reduced or the packs life will be significantly compromised.

Rule of thumb / best practice: For best performance only push a cell to 50% of its stated maximum discharge capacity. If you are using a 10a cell, you should restrict current to 5a per cell. In the context of a complete pack, multiply 50% discharge capacity by the number of parallels to get your conservative maximum. 

Discharge rating of battery = (0.5 * X) * Y

where X = cells stated maximum discharge (refer to spec sheet)

Y = the number of parallels in your pack. 

Regarding the 4p packs commonly used upto 30a you are better to go for a higher discharge cell rather than the highest capacity cell. While on paper the 14Ah pack might look better, under real world conditions a pack that is spec’d with a 3000mAh higher discharge cell (making the pack 12Ah) will get you similar riding range.

The Transfer of Energy Through the Pack

Current transfer through the battery is also very important. You want to ensure none of the nickel in the pack is heating up more than it should. Going back to the image earlier from electricbike-blog.com it appears that current is not flowing equally across the cells as the builder has only used two nickel strips to connect the series. At standard thickness used in ebike packs the nickel is only good for around 10a. So you need more than one connection between the series if you want to push to 30a or the strip will get very very hot.

Image: Possibly the worst pack design we have seen. Note the corrosion due to cheaper steel tabs being used. This is from a well know supplier.

Image: How the strips should look on the earlier pack for more even current transfer between the series. Source endless sphere user Allex.

Gallery Of Batteries we have seen in workshop over past 3 years. Some a serious safety risk.

To demonstrate a good current transfer please refer to the image below. This is using readily available cell holders and pre-fabricated strips which are a good option for DIY guys wanting to build a reliable pack.

Image: Very simple configuration showing good connection between the the series.

So what else goes into a good pack?

Over the past few years we have been assessing and repairing batteries from many sources, the majority of which are manufactured in mainland China. Quality vary’s wildly, and its almost impossible to tell the good from the bad when sorting through the myriad of suppliers on Alibaba / Aliexpress. Recently we were in Changzhou China where we visited Paul Lynch at the EM3EV battery factory. Paul has a solid engineering background and he has gone to great lengths to set-up a high tech factory. Paul’s factory is small compared to other mainland factories, but he has done this so QA remains strict. The cell holders are CNC’d in-house and customised to the shape of the pack. The unique holder and nickel tab design of the frame packs is shown in the image below.

What struck us initially about this particular pack was the dedication to safety in particular:

  • Poly-fuses are used on the packs main charge and discharge cables protecting the BMS from short circuits or any voltage spikes from the charger.
  • Poly-fuses are used on each of the BMS wires connected to the positive of each series.
  • Each cell has a fusible link via the nickel tab meaning if there is a problem with the cell it isolates from the rest of the pack limiting damage.
  • The CNC cell holders give the pack great strength and there is adequate spacing between the cells for cooling.
  • High discharge cells are used in this 4p pack meaning the pack can be conservatively rated for applications upto 30a. Each parallel is rated to 20a so at 30a we are still below that conservative limit.  Heat will be lower as the cells will not be significantly stressed.
  • The pack uses modular plugs to connect to the discharge port, the charge port and the switches so if/when these items need to be serviced in the future it is an easy job.

Conclusion

Do not be afraid to dig deep when buying a lithium-ion battery to ask whether they use cell holders, whether they use pure nickel strips and whether they are using the correct cell for your discharge requirements. Safety is the most important thing, so the more information you can gather the better.

In part 2 we will be presenting some in-house data from our own testing showing cells heating up under discharge and the heat differences between a pack with good build quality and bad. We will also present a simple look-up-table you can use to make sure you choose the correct battery for your motor / controller combination.

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